Consider updating your motherboard BIOS. This might give a minor boost to the system. Run spyware removal programs, defrag, and perhaps kill some of the programs that is automatically loading up on startup. You can run the msconfig tool in the run menu to prevent some of these programs from starting up. I find that when my computer that holds most of my music files is running slow, its usually infested with spyware or programs that start up automatically. I run spyware removal programs, uninstall unnecessary programs, prevent some program from automatically starting up, and finally defragging. It runs pretty well after that. You might want to make your swap file (the place that windows reserve for virtual memory) a constant 1GB so that Windows doesn't expand it too high and then when it decides to reduce the size, bring your system to its knees. You can find this area in the advance tab of the system properties menu. Click on the settings button for the performance. Look for the virtual memory settings area and make a custom size and set the initial and maximum size to 1GB. I do this for all of my systems and it seems to keep my system from getting periodic slow downs.
If you got the cash, get a faster SATA drive and make it your primary drive for running applications and use your current drive for data storage. Also, getting your system RAM to 1GB makes a world of a difference. Beyond that, its overkill.